1. Field of the Invention
The present invention relates to a power unit for a small vehicle. The unit composed of an internal combustion engine with a crankshaft and a power transmission system. The power transmission system is equipped with a belt-type continuously variable transmission, having a drive shaft and a driven shaft, which are arranged in parallel to the crankshaft. The belt-type continuously variable transmission is disposed below the axis of the crankshaft. The torque from the crankshaft is transmitted to the drive shaft so as to make the crankshaft and the drive shaft rotate in opposite directions. The power transmission system also includes an output shaft, which is linked and interlocked with the driven shaft via a gear transmission system, and which is disposed above the continuously variable transmission.
2. Description of Background Art
Japanese Utility Model Registration Application Publication No. SHO64-55196 discloses a conventionally known example of such a power unit as the one mentioned above. In the power unit of this disclosure, the crankshaft, and the drive and the driven shafts of the belt-type continuously variable transmission are arranged in a triangular shape in a projection drawing on a plane that is orthogonal to these axes, with the crankshaft being positioned at the apex. In addition the output shaft having a gear that meshes with another gear provided on the driven shaft is disposed adjacent to the driven shaft.
In the power unit disclosed in Japanese Utility Model Registration Application Publication No. SHO64-55196, the driving power is transmitted from the crankshaft to the output shaft via the drive and the driven shafts of the belt-type continuously variable transmission. The crankshaft and the output shaft rotate in the same rotational direction. When a gear transmission replaces the continuously variable transmission, the driving power is transmitted from the crankshaft to the output shaft via the main and the counter shafts of the gear transmission, and the crankshaft and the output shaft rotate in rotational directions opposite to each other. Accordingly, when the power unit is equipped with a gear transmission instead of a belt-type continuously variable transmission, not only the power transmission system but also other components in the drive train from the output shaft of the power unit to the drive wheel have to be changed.
In this respect, a possible solution is to make a gear reduction mechanism that is provided between the driven shaft of the continuously variable transmission and the output shaft be so configured that the driven shaft and the output shaft can rotate in the same rotational direction. In this case, however, it is desirable that the space to dispose the gear reduction mechanism be secured so efficiently that the power unit may be prevented from becoming larger in size.